1. Field of the Invention
The present invention relates broadly to the surface treatment of solids and more particularly to the recycle of rubber products and still more particularly to the use of recycled crumb rubber.
2. Discussion of Background
Existing efforts to recycle used rubber, in particular used tires, into new rubber articles, especially tires, have met with only limited commercial success. In the United States alone, there are currently billions of tires stock-piled in long term storage with additional millions being added annually to such stock piles. Because of the large volume involved with tires, this discussion will be directed to tires although much of these comments are applicable to other new and used rubber products. Those skilled in the art will know when the comments are appropriate to other new or used rubber products. Also, because used rubber is usually processed in the form of crumb rubber, references herein will be to rubber in that form. A major limitation in the recycling of used tire material is that the used tire rubber can not be readily mixed in economical proportions to form suitable new tire polymer mixes.
During the vulcanization process of new tires, accelerators, promoters, and/or initiators, are used which result in the formation of large numbers of sulfur crosslinks in the resulting vulcanized tire rubber. It is generally believed in the art that the sulfur compounds which are present in used tire rubber are highly reactive in the tire curing process such that when used tire material is mixed with new tire polymer, the used material is too reactive. Reformulation of used tire rubber with virgin materials results in a brittle compound unsuitable for many uses and among them are vehicle tires such as automobile or truck tires. Heretofore, efforts to reclaim scrap rubber have included a physical sheering process which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber. Such use is taught in U.S. Pat. No. 5,304,576.
It is also known to take used rubber and depolymerize the vulcanized rubber in an organic solvent and then recover various polymerized fractions as taught in U.S. Pat. No. 5,438,078. Similarly, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds.
U.S. Pat. No. 4,161,464 teaches a method to devulcanize rubber from commercial scrap rubber in which the vulcanized particles of used tires are reacted with an organic solvent to swell the rubber particles. Following swelling, the vulcanized particles are reacted with an aqueous solution of OH.sup.- ions. The ions disrupt polysulfide crosslinks which produces a rubber product better able to be recycled.
U.S. Pat. No. 5,602,186 to Myers et al., recites a process for devulcanizing a rubber crumb by introducing an alkali metal in the presence of a solvent to break polysulfide crosslinkages. The resulting devulcanization process is used to liberate rubber polymer
There remains a strong need for a practical, economical system for processing used rubber into a material which can be incorporated at a substantial loading level into new rubber compounding mixtures. There also remains a dire if not critical need for a practical, economical system for processing used tires into a material which can be incorporated at a substantial loading level into new tire compounding mixtures. Preferably, such a system could be operated under ambient conditions without the use of hazardous reactants or solvents.